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34 • Astronomy Basics
Figure 4-3
When the telescope’s axis of rotation is parallel to the Earth’s axis, stars viewed
through the telescope appear stationary when using a motor drive.
In order for the telescope to track the stars it must meet two criteria. First,
you need a drive motor that will move at the same rate as the stars. For the G-8N
there are two optional motor drives (#93518 and #93523) that can be fitted to it. For
the C150-HD there are also two optional motor drives (#93517 and #93522). The
second thing you need is to set the telescope’s axis of rotation so that it tracks in the
right direction. Since the motion of the stars across the sky is caused by the Earth’s
rotation about its axis, the telescope’s axis must be made parallel to the Earth’s axis.
Polar alignment is the process by which the telescope’s axis of rotation is
aligned (made parallel) with the Earth’s axis of rotation. Once aligned, a
telescope with a clock drive will track the stars as they move across the sky.
The result is that objects observed through the telescope will appear stationary
(i.e., they will not drift out of the field of view). If your telescope does not use a
motor drive, all objects in the sky (day or night) will drift out of the field. This
apparent motion is caused by the Earth’s rotation. Even if you are not using a
motor drive, polar alignment is still desirable since it will reduce the number of
corrections needed to follow an object and will limit all corrections to one axis
(R.A.). There are several methods of polar alignment, all of which work on a
similar principle, but are performed somewhat differently. Each method will be
considered separately, beginning with the easier methods and working to the
more difficult, but more precise.
Although there are several methods mentioned here, you will never use all of
them during one particular observing session. Instead, you may use only one
if it is a casual observing session. Or, if you plan on astrophotography, you
may use two methods — one for rough alignment followed by a more accurate
method.
Definition: The polar axis is the axis around which the telescope rotates when moving the
telescope in right ascension. This axis remains stationary as the telescope
moves in right ascension and declination.
Polar Alignment